The present invention relates to a coating method generally called free-falling vertical curtain coating, and in particular, to curtain coating used in manufacture of photographic light-sensitive materials.
As a technology to form layers of various types by coating some emulsions on a support (paper, metal, synthetic resin or the like) conveyed continuously, there are given a slide bead coating method, a rod coating method, an extrusion coating method and a curtain coating method. In the method called the curtain coating method among the aforesaid methods, a coating solution flowing out of slits provided on the slide surface flows down uniformly along the slide surface, then, goes down in the state of a curtain formed between edge guides provided on both sides of the lower part on the slide surface to fall on the support so that a coating layer is formed. When comparing with a conventional slide hopper coating method, for example, this curtain coating method can coat thinner layers at higher coating speed, and it is superior to the slide bead coating method in terms of mass production and of energy conservation.
On the other hand, however, there is caused an edge unevenness tendency in the curtain coating method, because "a process to form a coating solution to be curtain-shaped" is newly added. This is a tendency wherein a thickness of an edge portion in the conveyance direction on a layer coated and formed is increased, resulting in uneven layer thickness of a total coating layer. With regard to this point, various studies which have been made up to now show that unevenness of the curtain thickness in the lateral direction is increased as the curtain flows down between edge guides.
In addition, a flow speed of the curtain (coating solution) is especially slow in the vicinity of an edge guide, which sometimes causes destruction of an edge portion on the curtain.
Namely, there is required, for the appropriate curtain coating, a technology to keep the layer thickness constant in the lateral direction while forming the curtain itself stably,
For these requirements, there have been suggested technologies disclosed in (1) TOKKAISHO 51-57734 and (2) TOKKAIHEI 4-354563, for example. However, the technology in (1) above has a problem that unevenness in the lateral direction increases, though the curtain itself therein is stabilized. This problem is remarkable especially when a coating solution having low surface tension is used. In the case of the technology in (2) above, an effect is insufficient totally, and in particular, when the flow rate of a coating solution is lowered, unstable and uneven portions on the curtain are increased.
Therefore, there have been suggested technologies relating to the so-called "side solution" wherein a solution which is different from the coating solution is caused to flow down along the edge guides to support a curtain, such as those disclosed in (3) TOKKAISHO 59-132966, (4) TOKKAIHEI 3-50085, (5) TOKKAIHEI 1-199668 and (6) TOKKAIHEI 5-293429.
Even in these technologies, however, a problem of "unevenness in the lateral direction" has not been solved sufficiently, though they are effective on the point of "forming the curtain stably". When the flow rate of the side solution is great, in particular, mixture of the side solution and the coating solution is caused in the vicinity of the edge guide, resulting in an increase of "unevenness in the lateral direction", which is a problem.
This unevenness of a layer thickness on the edge portion is one shown in FIG. 6, and proposed technologies for letting the side solution flow down as stated above, for absorbing the coating solution on the edge portion after coating, and for improving physical properties of the coating solution, have not yet solved the problem of unevenness of a layer thickness effectively, and these technologies can not cope with high speed coating presently.